Re: [Phys-l] g...

[Ludwik Kowalski <kowalskil@mail.montclair.edu>]

On Nov 18, 2006, at 9:35 PM, Jack Uretsky wrote:

>  	I don't see the benefit of starting with acceleration as opposed
> to, for example, mass.  Once you have persuaded your students to
> differentiate mass from weight, then weight per unit mass (as a proxy 
> for
> F/m) can follow directly.  It is inntuitively and experienbtially 
> obvious
> that it is harder to push a truck, than a wheelbarrow - the essential
> difference is mass, not weight.  The notion of field is not necessary 
> at
> this point, so I don't see that F/m is all that difficult.

The advantage of starting with kinematics, and thus with acceleration, 
is that only time and distance are involved. These are usually not 
confused by students.
It also gives a teacher time to evaluate mathematical sophistication of 
students, before choosing a strategy for dealing with F=m*a. My 
strategy, in a non calculus university course was to begin by saying 
that  F is determined by what a horizontal force-meter, calibrated in 
N, shows. My demo force-meter had a large circular scale with a visible 
indicator. F can be a weight (applied via a pulley) or it can be a 
strong man pulling, wind blowing on a sail of a boat or a magnet 
opposing my attempt to separate it from a piece of iron. The class 
activity goal was to make clear that the concept of force is not 
limited to weight. Weight is just one of many different forces. If the 
mass is also introduced independently of the F=m*a, then students might 
be led to discover the second Newton's Law experimentally, for example, 
via measuring a=9.8 and showing that it happens to be close to F/m. Or 
from experiments with Pasco frictionless carts, etc.

Ludwik Kowalski
Let the perfect not be the enemy of the good.